1. Field of the Invention
The present invention relates in general to switched mode electronic circuits and in particular to circuits and methods for output impedance matching in switched mode circuits.
2. Description of the Related Art
Class D audio power amplifiers (APAs) have been used for many years in systems, such wireline telephony, where high bandwidth is not critical. More recently however, new fabrication techniques, and in particular, new techniques for fabricating power transistors, have made integrated Class D APAs possible. This has extended their potential applications to lower-power, higher-bandwidth systems, including battery-powered portable music players and wireless communications devices.
One major advantage of Class D amplifiers is their efficiency. Generally, an audio signal is converted into a relatively high frequency stream of pulses varying in width with the amplitude of the audio signal. This pulse width modulated (PWM) signal is used to switch a set of power output transistors between cutoff and saturation which results in efficiencies above 90%. In contrast, the typical Class AB push-pull amplifier, using output transistors whose conduction varies linearly during each half-cycle, has an efficiency of around 60%. The increased efficiency of Class D amplifiers in turn reduces power consumption and consequently lowers heat dissipation and improves battery life.
Given the importance of improved battery-life, reduced heat dissipation, and component size minimization in the design and construction of portable electronic appliances, improved switched mode techniques will have numerous practical advantages. The possible applications for these techniques are numerous, although Class D APAs are one of the primary areas which should be considered.
According to one embodiment of the principles of the present invention, a switched mode output stage is disclosed which includes first and second output transistors for respectively driving an output terminal from the first and second voltage rails in response to a driving signal and a complement of the driving signal. First and second reference transistors are included having current paths coupled in series at a node and selectively coupling the first and second voltage rails, the reference transistors scaled with respect to the first and second output transistors. Measurement and control logic includes circuitry for sensing an imbalance between a voltage appearing at the node and a reference voltage and feeding back a control signal to a gate of a selected one of the reference transistors to compensate for the imbalance. Additional circuitry varies and impedance of a selected one of the first and second output transistors in response to the control signal.
Circuits and methods embodying the inventive concepts advantageously provide means for matching output impedance. Among other things, these techniques can be used in switched mode circuits, such as switched mode amplifiers, and similar circuitry. Implementing the inventive principles is efficient, from both a cost and circuit performance point of view.